sysbuild: set APPLICATION_BINARY_DIR to sysbuild CMake folder

If not defined by caller, then set APPLICATION_BINARY_DIR to
CMAKE_CURRENT_BINARY_DIR. This is normally be done by
`find_package(Zephyr)`, but due to the indirection introduced with
fc1884ec to allow applications to
provide their own sysbuild entry point, then the APPLICATION_BINARY_DIR
is unexpectedly set to the bin dir of the entry point.

Restore the old behavior so that APPLICATION_BINARY_DIR per default
again points to top-level sysbuild binary dir.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>